Here is how I have been controlling temperature for bulk fermentation. I use an Igloo MaxCold 120qt cooler with a ~64oz bottle of ice. Any sufficiently sized and well insulated cooler should work fine. I keep a couple extra bottles of ice in the freezer to change out with the one in the cooler on a regular basis (usually 12 hours).

Before doing the left and right side tests described below, I kept the dough (in the red-top Rubbermaid container) on the far right side of the cooler. I now keep it closer to the center as shown in the picture below.

The temperature profile is the temperature directly above top of the red-top container positioned in the center as shown below. The container was filled with 1.5kg water at 77F to simulate a dough mass. The ice bottle was changed 3 times at approximately 12 hour intervals. The green (ambient) line represents the ambient temperature (set to 75F) outside the cooler. You can see the air conditioner turning on and off to maintain the temp. The two extended periods when it was off were nighttime.

Here are the results of two (~24 hour) tests set up as shown in the picture below. The first temperature profile shown was with 1.5kg of 77F water placed in the Rubbermaid container on the right. The second profile is with 1.5kg of 77F water in the container on the left. The temperature profile is the temperature profile directly above the two red-top boxes. You can see the temperature probes in the picture. There was no ice change in these tests.

Here is a combination of three three charts above that shows the temperature directly above the 1.5kg of 77F water for the first 22 or so hours at all three locations. The drop around 12 hours in the center location profile was an ice bottle swap.

Great post, Craig. Temperature control within a few degrees can make all the difference in the world for long fermenting of sourdough. Your simple, effective, and inexpensive system should allow any sourdough baker to get the very best out of their starter.

Craig, in this setup I see your probes and graphs. You mention an AC which comes on/off. How is that controlled relative to the probe temp readings? You put about 1.5 liters of water in the bottom (I didn't see that in your photo)?

The AC is my home AC - you see it in the ambient temp (outside of the cooler) - not the cooler temp. The 1.5kg water is in the containers to simulate having a mass of dough in the cooler.

Ahhh....the water was for simulation. Very clear now.....I was thinking you had the containers floating in shallow water pool to aid in maintaining temp. I have a Igloo that's been sitting around....so looks like a good option. Harder to figure this out given it's winter up here in CT now.

First I tried the Brod and Taylor Folding Proofer. It turned out to be a floater. At that point I was ready to go DIY but at the last minute I decided to give this a shot and I'm glad I did. It'll comfortably hold 8 of these and it has no problem maintaining 58 degrees in a 72 degree room.

First I tried the Brod and Taylor Folding Proofer. It turned out to be a floater. At that point I was ready to go DIY but at the last minute I decided to give this a shot and I'm glad I did. It'll comfortably hold 8 of these and it has no problem maintaining 58 degrees in a 72 degree room.

We think alike. I was looking at our wine fridge about that size but decided that there was over $1000 in wine in it and the weather was hot, so no way I was going to jeapordize that wine for some dough. I may buy another cooler that size and it should free up a 12 bottle fridge for my dough.